Currently, a variety of sensor means are used in vehicles. These sensor means are used on the one hand to detect surrounding objects, this detection being carried out for example by ultrasound sensors, and are used on the other hand for vehicle impact recognition, by detecting high acceleration values using acceleration sensors.
Additional sensor means in vehicles include sensors for recognizing environmental quantities, such as recognizing rain, measuring temperatures, determining adhesive friction between the vehicle and the roadway, measuring intrinsic speed, and measuring ambient brightness.
Currently, ultrasound sensors in vehicles are used exclusively for convenience functions, such as providing assistance when parking. In this way, ultrasound sensors are used to acquire distances and changes in distances from objects situated in the immediate vicinity of the vehicle.
Evaluation of the ultrasound signals makes it easier for the driver to estimate the distance from the surrounding objects, so that an impact can be avoided, especially at low speeds.
It is to be expected that ultrasound sensors will be used for many functions in the future, because they are inexpensive. Thus, in many vehicles ultrasound sensors will become standard equipment. Because in principle the ultrasound sensors can be used to detect an impact, it is desirable also to use such ultrasound sensors to recognize the threat of an impact. In terms of percentage, the greatest number of accidents take place at a relative speed between the vehicle and the surrounding object that lies within the measurement range of the ultrasound sensors, so that the ultrasound sensors can be used for impact recognition. However, because a recognition of a vehicle's impact with an obstacle is a highly safety-relevant process, the sensor signals delivered by ultrasound sensors must have an extremely high degree of reliability.
Disadvantageously, ultrasound sensor systems currently supply false signals, or no signals at all, when the environmental influences acting on the ultrasound sensors are not favorable. For example, there is the danger that sensors used near the roadway can become dirtied by rain or sprayed water, thus supplying false distance values, or no distance values. At low temperatures, together with rain and snow, ultrasound sensors of this sort often become covered with ice, which also greatly limits the reliability of the sensor signals that they deliver.
A further disadvantage of ultrasound sensor systems is that for physical reasons the ultrasound requires a medium of propagation, namely the air surrounding the vehicle. In a runtime measurement using ultrasound sensors, which is used in order to recognize a relative speed between the vehicle and the obstacle, a significant role is played by the movement of the air against the sensor, for example the wind, at a high intrinsic speed. For the reasons cited, up to now it has not been possible to use the existing ultrasound sensors in vehicles for the recognition of the impact of a vehicle against an obstacle.